The present invention relates to an apparatus for separating solvents having a limited degree of miscibility. In particular, the present invention relates to an apparatus for the removal of water from glycol ether dry cleaning solvents. The present invention further relates to a system in which the apparatus of the present invention is used in combination with a dry-cleaning machine, which cleans fabrics and other materials with a glycol ether dry cleaning solvent, to remove water that accumulates in the solvent during the dry cleaning process.
In a dry-cleaning machine, the clothing or other fabric to be cleaned is tumbled or agitated in the presence of a liquid solvent that removes dirt, oil, grease and other soiling substances from the fabric. Garments entering a dry cleaning plant contain significant quantities of water in the form of moisture. The water is removed from the fabric by the solvent along with the soiling substances.
Traditionally, dry cleaning solvents, such as perchloroethylene, are water-immiscible and have a density greater than that of water. Thus, when the perchloroethylene is returned to the solvent tank of a dry cleaning machine, the water removed from the fabric floats to the surface, with any soil dissolved therein, where it is easily removed. The other soiling substances are removed either by filtration or, alternately, by distillation of the dry cleaning solvent.
Because perchloroethylene poses a hazard to health and the environment, substitute solvents have been developed. EP 479,146 discloses the use of propylene glycol monomethyl ether as a safe alternative to perchloroethylene. The use of propylene glycol tertiary butyl ether (PTB) and propylene glycol n-butyl ether (PNB) as dry cleaning solvents is disclosed by WO 98/45523. Other glycol ethers have been identified as potential replacements for perchloroethylene as a dry cleaning solvent. The glycol ethers, especially PTB and PNB, possess the requisite detergency for dry cleaning without damaging garments and other fabrics. The glycol ethers also dry at temperatures suitable for use with fine fabrics. From the standpoint of health and safety, glycol ethers are non-carcinogenic, nontoxic and biodegradable.
Glycol ethers also differ from perchloroethylene by being marginally miscible with water, particularly at the temperatures employed with dry cleaning. Consequently, the glycol ether is diluted by clothing moisture during the dry cleaning process, reducing the cleaning ability of the solvent. This can be restored by replenishment of the glycol ether through distillation.
WO 98/45523 discloses that water can be removed from PTB and PNB by distillation. However, the energy required for distillation of glycol ethers is also costly. Ideally, distillation should be reserved for reclaiming heavily soiled solvent.
U.S. Pat. Nos. 3,674,650; 5,069,755; and 5,236,580 disclose distillation systems for use with dry cleaning machines to purify perchloroethylene. U.S. Pat. No. 4,191,651 discloses an apparatus for separating two immiscible liquids. However, such a device would not efficiently separate liquids having even a limited degree of miscibility.
There remains a need for an energy efficient means by which water can be separated and removed from modem glycol ether dry cleaning solvents.
This need is met by the present invention. The present invention provides an apparatus for separating two miscible liquids with a high degree of energy efficiency. The apparatus cools the mixture to a temperature below which the two liquids are miscible and then employs gravity separation to partition the two liquids. The apparatus may be used to separate essentially any mixture of two miscible solvents.
Therefore, according to one aspect of the present invention, an apparatus is provided, including:
a vertical accumulator tank having a top end and an bottom end, with the accumulator tank being serially connected to a vertical reservoir tank having a top end and a bottom end;
a down tube vertically positioned in the accumulator tank and having a top end nearest the top end of the accumulator tank and a bottom end nearest the bottom end of the accumulator tank, and with an inlet at the top end of the down tube and an outlet at the bottom end of the down tube;
an inlet conduit connected to the inlet of the down tube, the inlet conduit entering the bottom end of the accumulator tank and defining an upward flow path within the accumulator tank to the inlet at the top end of the down tube;
a heat exchanger in communication with the down tube for withdrawing heat therefrom; and
a transfer conduit defining a downward flow path from the top end of the accumulator tank to the bottom end of the reservoir tank.
The apparatus is most effective for the separation of liquids that are miscible above and immiscible below a temperatures of about 60xc2x0 C. The apparatus can be used to separate liquid mixtures that remain miscible at temperatures as low as room temperature, so that the apparatus may be used to separate heated liquid mixtures that are immiscible at room temperature (room temperature defined as being about 22xc2x0 C.).
The apparatus is particularly useful in dewatering glycol ethers used as the cleaning solvents in modern dry cleaning equipment. Therefore, according to another aspect of the present invention, a system is provided in which a dry cleaning machine adapted to cleaning clothing or other fabrics with glycol ethers, and having a cleaning section in communication with a glycol ether storage tank, is combined with the apparatus of the present invention. Glycol ethers from the storage tank are pumped to the apparatus where they are dewatered. After dewatering, the glycol ethers are then pumped back to the solvent storage tank.
Some water will always be bound to the glycol ether as an azeotrope. However, the removal of excess water from the glycol ether is critical in order to control the shrinkage of woolens and other fabrics.
Typical dry cleaning systems have three or more solvent tanks, one of which is used to store reclaimed, distilled solvent, with the others being designated work tanks, which supply dry cleaning solvent to the cleaning sections of the dry cleaning machine, which then returns the solvent to the work tanks. The present invention therefore also includes an apparatus in which the accumulator tank is compartmentalized into individual cells, each dedicated to a separate solvent tank of a dry cleaning machine. Therefore, according to another aspect of the present invention, an apparatus is provided in which the accumulator tank is divided vertically into a plurality of isolated compartments, each compartment having a down tube vertically positioned therein. The apparatus further includes a plurality of inlet conduits, each entering the bottom end of an individual compartment and defining an upward flow path within each compartment and connected to the top inlet of each down tube. Each down tube is provided with a heat exchanger, and each compartment has a separate transfer conduit defining a downward flow path to the bottom end of the reservoir tank. The purpose of rising to the top and then back to the bottom is to limit the amount of loss from the cell should there be a failure of the input tube.
A single reservoir tank may be used to collect dewatered solvent from the plurality of accumulator tank compartments, which is then supplied to the plurality of dry cleaning machine solvent tanks from the single reservoir tank. Alternatively, the reservoir tank may also be divided vertically into a plurality of isolated compartments, each compartment corresponding to a compartment of the accumulator tank. Each of a plurality of transfer conduits would define a downward flow path from the top end of an accumulator tank compartment to the bottom end of the reservoir tank corresponding thereto. Each reservoir tank compartment is then dedicated to a solvent tank of the dry cleaning machine and each of a plurality of outlet conduits defines a flow path exiting the bottom end of its reservoir tank compartment and returning to the dry cleaning machine solvent tank corresponding thereto.
The apparatus of the present invention may also be used in other end use applications for glycol ethers in which dewatering is required. Such applications include, but are not limited to, dyeing processes in which glycol ethers are used as the solvents, processes for scouring raw wool and milling the scoured raw wool, and processes in which glycol ethers are used to clean or degrease metal parts or electronic components. Furthermore, the apparatus of the present invention may be applied to essentially any end use application in which it is desirable to obtain the separation of two miscible solvents.
The foregoing and other objects, features and advantages of the present invention are more readily apparent from the detailed description of the preferred embodiments set forth below, taken in conjunction with the accompanying drawings.